docs: def _initialize_disk_threads(self):
          # type: (Uploader) -> None
          """Initialize disk threads
          :param Uploader self: this
          """
          logger.debug('spawning {} disk threads'.format(
              self._general_options.concurrency.disk_threads))
          for _ in range(self._general_options.concurrency.disk_threads):
              thr = threading.Thread(target=self._worker_thread_upload)
              self._disk_threads.append(thr)
              thr.start()